The USA’s Ground-Based Midcourse Defense (GMD) program uses land-based missiles to intercept incoming ballistic missiles in the middle of their flight, outside the atmosphere. The missiles are currently based at 2 sites in the USA: 4 at Vandenberg AFB in California, and 20 (eventually 26) at Fort Greely in Alaska.
The well-known Patriot missiles provide what’s known as terminal-phase defense options, while longer-reach options like the land-based THAAD perform terminal or descent-phase interceptions. Even so, their sensors and flight ranges are best suited to defense against shorter range missiles launched from in-theater.
In contrast, GMD is designed to defend against intercontinental ballistic missiles (ICBMs). It depends on tracking that begins in the boost phase, in order to allow true mid-course interception attempts in space, before descent or terminal phase options like THAAD and then Patriot would be tried. In order to accomplish that task, GMD missiles must use data feeds from an assortment of long-range sensors, including satellites like SBIRS and DSP, some SPSS/BMEWS huge early-warning radars, and even the naval SBX radar.